I. Technical Field
This invention relates to a low-pressure steam turbine for use in thermal and nuclear power plants.
II. Description of the Related Art
A low-pressure steam turbine used in a thermal power plant or nuclear power plant is driven under wet steam condition in the vicinity of its final stage. Under the wet steam condition, there occurs wet loss, that is thermodynamic and hydrodynamic energy loss, along with generation or growth of drain, and the turbine efficiency is deteriorated. If the drain collides against turbine moving blades rotating at a high speed, the blade surfaces will be possibly subject to erosion, resulting in deterioration of reliability of the turbine.
As a measure for reducing wet loss and preventing erosion in a low-pressure steam turbine, a conventional technique is known in which drain is removed by means of a drain catcher or hollow stationary blades. As a technique of using a drain catcher in a low-pressure steam, Japanese Patent Application Publication No. 2001-55904 listed below, for example, discloses a technique in which a drain catcher is provided on a stationary blade outer ring supporting stationary blades. According to the technique disclosed in Japanese Patent Application Publication No. 2001-55904, drain contained in turbine driving steam is caught with the drain catcher, and the caught drain is discharged outside through a passage. Further, as a technique of using hollow stationary blades in a low-pressure steam turbine, Japanese Patent Application Publication No. H11-336503, for example discloses steam turbine stationary blades, in which each stationary blade has a cavity passing from an outer shroud to an inner shroud through the inside of the stationary blade. The stationary blade also has a plurality of slits which connect the front-side and back-side surfaces of the stationary blade to the cavity and extend vertically while being spaced from each other by a predetermined distance. The stationary blades of the steam turbine disclosed in Japanese Patent Application Publication No. H11-336503 are able to introduce drain into the cavities inside the stationary blades through the slits and collect the drain from the cavities.
Further, as another measure for reducing wet loss and preventing erosion, there is known a conventional technique in which stationary blades are heated by introducing steam into the inside of the stationary blades from the outside in order to prevent condensation of steam on the surfaces of the stationary blades. Japanese Patent No. 3617212, for example, discloses a technique of heating stationary blades, in which leakage steam at high temperature and low pressure is extracted from a shaft seal gasket upstream of a high-pressure stage of the turbine and is introduced into hollow stationary blades.
However, even though the technique of using a drain catcher as disclosed in Japanese Patent Application Publication No. 2001-55904 or the technique of using hollow stationary blades as disclosed in Japanese Patent Application Publication No. H11-336503 are able to realize reduction of wet loss and prevention of erosion by removing drain, they still have a problem that turbine driving steam may possibly be discharged together with the drain. As for the technique of heating stationary blades as disclosed in Japanese Patent No. 3617212, steam must be introduced from the outside as energy for heating the stationary blades. This means that the system as a whole requires introduction of energy from the outside. It may be also possible to heat the stationary blades with use of a heater instead of externally introducing steam. In this case, however, additional energy is required to drive the heater. Therefore, the system as a whole requires introduction of energy from the outside.